In a liquid crystal display device, to improve contrast of the display and to match color tone, it is required that the retardation film used in the liquid crystal display device fulfills the function enough for all incident light of a visible light range. That is, it is required that retardation of light having a short wavelength is small and retardation of light having a long wavelength is big. More specifically, it is required that a relation of R450<R550<R650 is satisfied, in which R450 is retardation of light having a wavelength of 450 nm at an incident angle of 0 degrees, R550 is retardation of light having a wavelength of 550 nm at an incident angle of 0 degrees, and R650 is retardation of light having a wavelength of 650 nm at an incident angle of 0 degrees.
In Patent Document 1, a retardation film which is manufactured by laminating a stretched film which is large in phase difference and small in Abbe number on a stretched film which is small in phase difference and large in Abbe number so that optical axes cross at almost right angles is disclosed as a film that satisfies a relation of R450<R550<R650.
Patent Document 2 discloses a retardation film which is manufactured by sticking a stretched film of which a ratio of retardation at a wavelength of 450 nm and retardation at a wavelength of 550 nm is 1.00 to 1.05 and a stretched film of which a ratio of retardation at a wavelength of 450 nm and retardation at a wavelength of 550 nm is 1.05 to 1.20. In these methods described in the Patent Document 1 or the Patent Document 2, an accurate axis match is required when films are stuck together.
Patent Document 3 and Patent Document 4 discloses a retardation film obtained by uniaxially stretching a laminated body composed of a layer of a resin having a positive intrinsic birefringence and a layer of a resin having a negative intrinsic birefringence, in which a direction of a molecular orientation of the layer of the resin having a positive intrinsic birefringence and a direction of a molecular orientation of the layer of the resin having a negative intrinsic birefringence are parallel.
Moreover, a retardation film which satisfies a relation of 0.92≤R40/Re≤1.08, in which Re is a retardation value at an incident angle of 0 degrees, and R40 is a retardation value at an incident angle of 40 degrees; and a retardation film which satisfies a relation of nx>nz>ny, in which nx is a refractive index in a direction of a slow axis in a plane of the film, ny is a refractive index in a direction perpendicular to the direction of the slow axis in the plane, and nz is a refractive index in a thickness direction are suggested to decrease dependence of color tone on an angle in a liquid crystal display device.
Patent Document 5 discloses that a first anisotropic film is obtained by uniaxially stretching a polycarbonate resin film; on the other hand, a second anisotropic film is obtained by uniaxially stretching a polystyrene resin film, and then the first anisotropic film and the second anisotropic film are overlapped so that the directions of stretching of the films are at a right angle to each other to obtain a retardation film which satisfies relation of nx>nz>ny.
In addition, Patent Document 6 discloses that a first anisotropic film is obtained by uniaxially stretching a polycarbonate resin film; on the other hand, a second anisotropic film is obtained by uniaxially stretching a polystyrene resin film, and then the first anisotropic film and the second anisotropic film are overlapped so that the directions of stretching of the films are at a right angle to each other to obtain a retardation film which satisfies a relation of (Re−Re40)/Re≤0.07. In these producing methods described in Patent Document 5 or Patent Document 6, an accurate axis match is required when films are stuck together.
Patent Document 7 discloses that a laminated body is formed by bonding a shrinkable film to one side or both sides of a resin film before the resin film is stretched, and then the laminated body is stretched and heated to apply contractive force to the laminated body in a direction orthogonal to the direction of stretching of the above-mentioned resin film to obtain a retardation film which satisfies a relation of 0<(nx−nz)/(nx−ny)<1.
In the producing method described in Patent Document 7, the contractive force should be controlled accurately.
Patent Document 8 discloses that a rod is obtained by extrusion of a melted polycarbonate resin, then a disc is obtained by slicing up the rod in round, a rectangular plate is obtained by cutting the disc, and then the rectangular plate is uniaxially stretched to obtain a retardation film which satisfies a relation of 0.92≤Re40/Re≤1.08. However, it is difficult to manufacture the retardation film having a large area by the method described in the Patent Document 8.